In conventional chromatographic processes, a pulse of a feed mixture and a continuously flowing carrier fluid are introduced into a column packed with an adsorbent. The adsorbent is usually a porous or granular solid or an inert granular material on which there has been deposited a film or coating of a desired non-volatile liquid adsorbent. The components of the feed mixture pass through the column with the carrier fluid at different velocities which are dependent upon the respective affinity of each component with the adsorbent. Therefore, each component of the feed mixture will be retained by the adsorbent for a time which is characteristic to it and will exit from the column with the carrier fluid at different times, the components of the feed that are least adsorbed in the column exiting first.
Normally, conventional chromatographic processes are conducted as batch operations. However, numerous attempts have been made to effect chromatographic separation of a multicomponent feed continuously (see Sussman, M. V. and N. S. Rathore, Chromatographia, Vol. 8, February 1975; Rendell, M., Process Engineering, April 1975; and Sussman, M. V., Chemtech, April 1976). One of the earliest techniques suggested involved filling the annular space between two concentric cylinders with adsorbent and passing the feed mixture and carrier gas through the adsorbent axially as the annulus rotated slowly (see Martin, A. J. P., Disc. Faraday Soc., Vol. 7, p. 332 (1949)). Another technique required a Gatling gun arrangement of vertical columns in a circle (see Svensson et al, Science Tools, 2, (2), p. 17 (1955). While several variations of the rotating annulus configuration have been suggested (see, for example, U.S. Pat. Nos. 3,187,486; 3,257,781; 3,511,028; 3,732,982; Dinelli, D. M. et al., J. Chromatography, 7, 442 (1962); Polezzo, S. and M. Taramasso, J. Chromatography, 11, 19 (1963); Taramasso, M. and D. M. Dinelli, J. Gas Chromatography, 2, 150 (1964); Wankat, P. C. et al, I&E.C. Fund., 15, No. 4 (1976); Scott, C. D., et al., J. Chromatography, 26 (1976)), all are mechanically complicated and cumbersome due to the difficulty in rotating the large number of tubes or the bed of solids. In addition, the efficiency of such devices is limited because, as in conventional packed chromatographic columns, reducing the particle size of the adsorbent causes an increased pressure drop in the column.
Several other geometries have been suggested for effecting continuous multicomponent separations. For example, U.S. Pat. No. 3,078,647 discloses a packed bed annular system wherein the feed mixture and carrier gas flow radially from the inner to the outer cyclinder while the cylinders rotate (see also U.S. Pat. No. 3,527,350). As another example, U.S. Pat. No. 3,503,712 discloses a system wherein two rotating discs are placed close together, each disc being coated with a thin layer of adsorbent solvent. No solid adsorbent is packed between the discs. Carrier fluid is injected at the center of the discs and flows in the radial direction while feed is introduced at a single point such that transverse flow will occur due to the angular motion of the discs and the radial flow of the carrier fluid. Yet another approach involves the alternate continuous injection of two carrier fluids into a rectangular slab packed with adsorbent, the flow of each carrier fluid being perpendicular to the flow of the other, such that the components in the continuously injected feed are separated as the direction of fluid flow changes at right angles while changing the temperature (see U.S. Pat. No. 3,482,376 and Tuthill, E. J., J. Chrom. Sci., 8 (1970)). However, packed bed arrangements suffer from the pressure drop limitation mentioned previously while disc systems are limited to handling low capacities.